Gearing, speed and efficiency

[Tin Pot]

ultimately my question is "which cassette, 11-25 or 12-27?"

http://www.joefrielsblog.com/2010/08/coast-or-pedal-on-a-downhill.html

According to the above link, for TTs and Ironman you want to be coasting and tucked at >50kph

According to the below link on a 50/34 crankset, 100rpm on 12 is just over 50kph, vs 90rpm on 11.

http://www.bikecalc.com/speed_at_cadence

So I'm in mind to get the 11-25 so that top speed feels easier, but the trade off is losing the spinning 34*27 that I may be desperate for after six or seven hours around Bolton.

...Which cassette?

 

[Racing roadkill]

ultimately my question is "which cassette, 11-25 or 12-27?"

http://www.joefrielsblog.com/2010/08/coast-or-pedal-on-a-downhill.html

According to the above link, for TTs and Ironman you want to be coasting and tucked at >50kph

According to the below link on a 50/34 crankset, 100rpm on 12 is just over 50kph, vs 90rpm on 11.

http://www.bikecalc.com/speed_at_cadence

So I'm in mind to get the 11-25 so that top speed feels easier, but the trade off is losing the spinning 34*27 that I may be desperate for after six or seven hours around Bolton.

...Which cassette?

50-11 ( fairly typical of a compact's big ring, with an 11 tooth sprocket) is nearly 120 gear inches. Not many people would need that. But I prefer the 11-28 Shimano cassettes, because I find that the steps between the sprockets gives more versatility.

 

[Citius]

So I'm in mind to get the 11-25 so that top speed feels easier

That's not how it works. You still have to put the effort in.

 

[Tin Pot]

50-11 ( fairly typical of a compact's big ring, with an 11 tooth sprocket) is nearly 120 gear inches. Not many people would need that. But I prefer the 11-28 Shimano cassettes, because I find that the steps between the sprockets gives more versatility.

Most of us spin out, even if only on the downhills. I'd just prefer to be getting to 50kph at a lower rpm.

My cassette is Veloce, hence the size options.

 

[Prometheus]

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I still use a 52t ring if possible, a most versatile ratio

then you can use the 12t-27t close ratio cassette,

but same hight gear as the 50t-11t

or even; dare I say a 12t-28t

 

[screenman]

I would love to be coasting at 30mph in whatever gear.

 

[ayceejay]

I think you might be mis reading Joe Friels advice Tinny as the two scenarios you mention are not related.

 

[Tin Pot]

I think you might be mis reading Joe Friels advice Tinny as the two scenarios you mention are not related.

Always happy to learn - could you point out what you think I've misread?

 

[ayceejay]

It seems to me that Friel is talking about strategy rather than speed per se and his advice is the same as for head winds. The question being do you tank it on the downhill/tail wind bit or save your energy for the uphill/head wind section. Since he advises tucking in at 50 mph when going downhill and not powering, the size of the cassette is not relevant. If however you are looking at the two things as not related I think you should choose a gearing set up that is suitable for the course.

 

[Tin Pot]

It seems to me that Friel is talking about strategy rather than speed per se and his advice is the same as for head winds. The question being do you tank it on the downhill/tail wind bit or save your energy for the uphill/head wind section. Since he advises tucking in at 50 mph when going downhill and not powering, the size of the cassette is not relevant. If however you are looking at the two things as not related I think you should choose a gearing set up that is suitable for the course.

Ah no, to get to 50kph will most likely require some pedalling.

Imagine a slight decline it's easier to pick up speed, you still want to be pedalling up until you're around 50 but you don't want your legs spinning like the clappers to get there.

 

[lukasran]

Def take the 27. Especially if you estimate 6 to 7 hour bike leg. It's a long day then a hellish run. Better save your legs. Crowds will be mega though.

 

[oldroadman]

The gear ratio at that point is not important, rpm is not that important. Air resistance increases at the square of the speed. (20 mph = 400 units, 30 mph = 900 units). Required wattage is what alters. Efficiency is at about 90-100 rpm. Pain is required to go fast, and practice spinning out on a low gear. Anyone who can't pedal at 40mph+ on a big gear is not flexible enough. Track wwork helps with leg speed and flexibility, or intervals at good pace on no more than 53x15 (lowish gear but should get well past 30mph).

 

[TheDoctor]

No-one ever needs to agonise over an 11-up cassette. If you're a pro, that's down to the mechanic and if you're not, you're not that good that you need it.
I'm no racer, and I can't spin out on 40 x 12. But it'll get me above 50 kph.

 

[Tin Pot]

Update.


Okay, so I did a hilly 10 mile TT today, and on the downhill I feel I definitely could've got more with a 53/11. After a long uphill, it feels better to be pushing a slower rpm/higher gear on the downhill.

Maybe it's slowtwitch/fast twitch, but it definitely feels better.

Uphill on TTs and triathlons are never huge gradients, so I'm thinking a standard crankset and an 11 smallest would help.

Tell me why I'm wrong

 

[Mr Bunbury]

I don't know about triathlons but unless you're doing some some specific 'Mountain TTs', you're bang on: 39x25 should be absolutely fine.

EDIT: Also, even if you're not pushing your 11 cog all the time, the higher gearing will mean that your chainline is more central and hence more efficient.